What is the best word to describe 14k+2 in 14k+2=16 ?

no, "14k + 2 = 16" is an equation

"14k + 2" is just an expression

You are correct, "14k + 2" is an expression, not an equation. Thank you for pointing that out.

The best word to describe 14k+2 in 14k + 2 = 16 is "equation".